The present invention relates to hydrodynamic bearings and disk recording/reproducing apparatuses equipped with them.
Disk recording/reproducing apparatuses include magnetic disks and magnetically or optically perform reading and writing of data for the magnetic disks while revolving the magnetic disks. Further increases in capacity and speedups of data transfers are required of disk recording/reproducing apparatuses. Accordingly, it is desired that revolutions of the magnetic disks become still faster and are stabilized with still higher precision. Hydrodynamic bearings are suitable for such high-speed and high-precision rotary drive systems.
An example of conventional hydrodynamic bearings is disclosed in the U.S. Pat. No. 5,433,529. FIG. 8 is a cross-sectional view showing the hydrodynamic bearing. The bottom end of a fixed shaft 22 is fixed on a base 21, and the top end is fixed on a cover (not shown). A flange 23 in an annular shape allows the top end of the fixed shaft 22 to pass through its inside and is fixed at the top end of the fixed shaft 22. A vertical groove 23C is provided on a side of the flange 23 touching a side of the fixed shaft 22, connecting spaces over and under the flange 23 to each other. Thrust dynamic pressure grooves 23A and 23B are provided on surfaces of the flange 23. A sleeve 24 and a hub 25 are integrated and surround the fixed shaft 22, being able to revolve around the fixed shaft 22. The flange 23 is then placed in a hollow 24D provided on an inner surface of the sleeve 24. A thrust plate 26 in an annular shape allows the top end of the fixed shaft 22 to pass through its inside, and is fixed at the top of the sleeve 24 and opposed to the flange 23. In this hydrodynamic bearing, in particular, a gap is provided between the top end of the fixed shaft 22 and the thrust plate 26. Radial dynamic pressure grooves (not shown) are provided on one or both of a side of the fixed shaft 22 and an inner surface of the sleeve 24. Radial dynamic pressure grooves are usually provided on two separate regions, a first region 24A near the flange 23 and a second region 24B near the base 21. Thrust dynamic pressure grooves and radial dynamic pressure grooves are, for example, herringbone-shaped grooves. Gaps between the fixed shaft 22 and the sleeve 24, in particular, the radial dynamic pressure grooves and their vicinities 24A and 24B, and the thrust dynamic pressure grooves 23A and 23B and their vicinities, are filled with a lubricant 27. Magnetic disks (not shown) are fixed on outer surfaces of the hub 25, being concentric with the fixed shaft 22. Magnets 28 are installed on inner surfaces of the hub 25. On the other hand, stators 29 are installed on the base 21 and opposed to magnets 28.
The above-described hydrodynamic bearing operates as follows. Rotating magnetic fields occur when the stators 29 are energized. The hub 25 undergoes a torque from the rotating magnetic fields through the magnets 28. Thereby, the sleeve 24, the hub 25, the thrust plate 26, and the magnetic disks (not shown) revolve in a body around the fixed shaft 22. During the revolution, the lubricant 27 flows along the radial dynamic pressure grooves and is concentrated in each central part of the first region 24A and the second region 24B. As a result, pressure in the radial direction of the fixed shaft 22 is enhanced in those central parts. This pumping effect maintains stable spacing between the fixed shaft 22 and the sleeve 24, and thereby the rotation axis of the magnetic disks does not substantially shift in the radial direction of the fixed shaft 22. Similarly, the lubricant 27 flows along the thrust dynamic pressure grooves 23A and 23B and is concentrated in each central part of regions where the thrust dynamic pressure grooves 23A and 23B are provided. As a result, pressure in the axial direction of the fixed shaft 22 is enhanced on surfaces of the flange 23. This pumping effect maintains stable spacing between the flange 23 and the hollow 24D of the sleeve 24 and stable spacing between the flange 23 and the thrust plate 26. Therefore, the axis of rotation of the magnetic disks does not substantially tilt from the axial direction of the fixed shaft 22. Here, the lubricant 27 is allowed to circulate on surfaces of the flange 23 through the vertical groove 23C of the flange 23. Accordingly, the lubricant 27 keeps covering the whole of the thrust dynamic pressure grooves 23A and 23B, even when shocks/vibrations act from the outside, for example, and therefore, the above-described pumping effects are not lost. Thus, the above-described hydrodynamic bearing maintains the high-speed revolution of the magnetic disks stable with high precision.
The lubricant 27 covers the whole of the radial dynamic pressure grooves and the whole of the thrust dynamic pressure grooves, for example, just after the lubricant 27 is poured into the above-described hydrodynamic bearing, and so on. Under such conditions, the above-described pumping effects are fully exerted. However, an abundance of air bubbles intrudes into the lubricant 27, for example, after a time lapse of use, and accumulates in and near the intermediate region 24C between the first region 24A and the second region 24B (see FIG. 8), for example. When those air bubbles swell with variations of outside air pressure or temperature rises of the lubricant 27, the lubricant 27 is pushed by the swelling pressure of the air bubbles and shifts in the axial direction of the fixed shaft 22. Thereby, the lubricant 27 tends to escape upward from the gap between the top end of the fixed shaft 22 and the thrust plate 26, and downward from the gap between the bottom end of the fixed shaft 22 and the sleeve 24 (see droplets 27A and 27B shown in FIG. 8). In the above-described hydrodynamic bearing, in particular, the vertical groove 23C is provided in the flange 23, and hence the lubricant 27 tends to rise through the vertical groove 23C and escape upward from the gap between the top end of the fixed shaft 22 and the thrust plate 26. Furthermore, a so-called lack of oil film, that is, a condition that the lubricant 27 fails to cover the whole of the radial dynamic pressure grooves and the thrust dynamic pressure grooves, occurs when the amount of leakage of the lubricant 27 is excessive. In that case, the above-described pumping effects become insufficient, and thus the risk of serious wear of the fixed shaft 22 and the sleeve 24 due to excessively hard contact between each other increases.